A method of this type is already known, for example from document EP-1 183 518, in which a coherent light beam, preferably a pulsed light beam, is injected into an optical fiber, via one end of the fiber, and said particles are illuminated by the pulses of the light beam emanating from the other end of said fiber.
To implement such a method, a monomode optical fiber is chosen for the purpose of preventing the occurrence of parasitic noise in said images.
However, to obtain contrasted sharp images of said particles, the energy injected at each pulse of the coherent light beam must be high enough to allow said particles to be illuminated sufficiently to form their images. In such a case when the injected light energy is high, despite the use of a monomode optical fiber, the images of said particles exhibit parasitic background noise and a moiré effect that degrade their sharpness.